In photographic industry, there have so far been the known couplers each capable of releasing a photographically useful group upon reaction thereof with the oxidized product of a developing agent. The above-mentioned photographically useful groups include, for example, a development inhibitor, a development accelerator, a bleaching accelerator, a fogging agent, a dye, and a fluorescent whitening agent. The above-given photographically useful groups are able to contribute to an image improvement and an image stability, when they are released imagewise or non-imagewise into a silver halide emulsion layer. For example, U.S. Pat. Nos. 3,227,554 and 3,148,062, and Journal of the American Chemical Society, Vol. 72, 1950, p. 1533, disclose each the couplers capable of releasing a development inhibitor or a dye from the coupling position. And, U.S. Pat. No. 3,703,801 discloses the couplers each capable of releasing a bleaching accelerator from the coupling position after completing a reaction of the oxidized product of a developing agent with the couplers. Further, there have so far been the known compounds each incapable of producing any dyes, but capable of releasing a photographically useful group upon reaction thereof with the oxidized product of a developing agent. Among these compounds, the hydroquinone capable of releasing a development inhibitor is disclosed in, for example, U.S. Pat. No. 3,930,863.
For the above-mentioned photographically useful groups including, particularly, a development inhibitor and a bleaching accelerator, it has been well known that the time and positional controls of displaying the advantages of the photographically useful groups shall be essential for the improvements of an image sharpness and an image graininess. With the purpose of controlling the advantages thereof, the couplers each comprising a photographically useful group coupled to the active site through a timing group are disclosed in U.S. Pat. No. 4,248,962. Besides, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 60-185950/1985 and 61 233741/1986 disclose the compounds each capable of releasing a photographically useful group, after the group is released from the active site of a coupler and then hydroquinone, catechol or pyrogallols each released thereby is then oxidation-reduction reacted successively with the oxidized product of a developing agent. In the compounds disclosed therein, however, the storage stability is poor and the unfavorable phenomenon is so produced as to be affected by an oxidation reaction before the coupler is to be reacted with the oxidized product of a developing agent. In this instance, when an aromatic compound is released, the readiness of the oxidation thereof shall of course be relative to the releasing speed, diffusing speed or its utility factor each of a photographically useful group. However, the readiness of the oxidation thereof may be controlled to some extent by adjusting the number of phenolic hydroxyl groups. In the second stage of the oxidation reaction of a compound substituted with a phenolic hydroxyl group in the position of a benzene ring, a desirable reaction speed may be obtained when 2 or 3 phenolic hydroxyl groups are substituted. In the form of releasing a photographically useful group from a conventional type coupler, there has raised a problem that a preserbavility is deteriorated by an air oxidation, because the hydroxyl group can protect only one coupler in the state where the hydroxyl group is coupled to the coupler. It may be expected that the deterioration of the preservability may be improved by protecting the phenolic hydroxyl group. However, the cleaving speed of a protective group has not been controlled in a procedure of hydrolyzing an ester bond, such as that disclosed in Japanese Patent O.P.I. Publication No. 61-230553/1986
Japanese Patent O.P.I. Publication No. 1-16494/1989 discloses the compounds each in which the hydroxyl group of a catechol is protected in the active site of a colorless coupler. However, those compounds have each a high pKa value in the active site position and incapable of releasing PUG sufficiently.
In the PUG-releasing couplers described in Japanese Patent O.P.I. Publication No. 1-154057/1989, the coupler reasing speed thereof was nothing but the same thing as in the conventional couplers capable of releasing PUG through a timing group.